Iron

Iron is the alchemists' Mars. The symbol is a circle with an arrow leaving it at the upper right, and it is still in use in biology as the sign for male. The associations are consistent across the tradition: war, blood, heat, the colour red, the sharpness of a blade. Rust is red, arterial blood is red, and the planet Mars is visibly red in the sky. The language kept the link long after the theory went: martial, martial law, and the iron in the blood, which is literal, since haemoglobin carries oxygen on an iron atom.

Before smelting there was meteoric iron. Iron arriving as a meteorite is already metallic and can be cold-worked, and objects made from it appear in archaeological contexts long before any furnace could reach the temperatures needed to reduce iron ore. Ancient Egyptian texts use a term conventionally translated as iron of the sky, and the iron dagger buried with Tutankhamun has been shown by X-ray fluorescence analysis to be meteoritic, matching the nickel and cobalt proportions characteristic of meteorites.

That is a genuinely important fact for a book of correspondences and it needs no embellishment. For a considerable period the only iron available anywhere was iron that had fallen from the heavens, and it was rarer and more valuable than gold. The association of the metal with the sky is older than its association with the planet Mars, and it rests on straightforward observation rather than on symbolism.

Smelted iron appears in the Near East during the second millennium BCE and becomes general after roughly 1200 BCE. Iron ore is far commoner than copper or tin, so once the technique existed, iron tools and weapons could be made in quantities that bronze never permitted. That is what the phrase Iron Age is describing.

Nuclear fusion releases energy when light nuclei combine, because the product is more tightly bound than the ingredients were. This holds up to a point, and then it stops. Binding energy per nucleon rises steeply from hydrogen and reaches its practical maximum around mass 56, at iron. Beyond that point, fusing nuclei together costs energy rather than releasing it.

A massive star therefore burns through successive shells, hydrogen to helium, helium to carbon and oxygen, and onward through neon, magnesium and silicon, each stage faster than the last, until it has built an iron core. At that point there is nothing left to burn. The core has no source of pressure to hold itself up, it collapses in a fraction of a second, and the star explodes.

Every element heavier than iron anywhere in the universe was made in such an event, or in the collision of neutron stars, by neutron capture rather than by fusion, and those processes are comparatively rare. This is why iron is common and gold is not, and it is a far better explanation of gold's value than any story about its colour.

Iron is the most abundant element in the Earth taken as a whole, roughly a third of the planet's mass, because it was abundant in the material the solar system formed from and because it is dense enough to have sunk. The core is iron with some nickel, liquid on the outside and solid at the centre, and convection in that liquid iron generates the magnetic field that shields the surface from the solar wind. The habitability of the planet rests on it.

Pure iron is unremarkable: soft, and it rusts. Almost everything useful about iron comes from what is dissolved in it.

Adding carbon, between roughly 0.02 and 2 per cent, gives steel, and the amount together with the heat treatment determines everything. Quenching hot steel in water traps the carbon in a distorted crystal structure called martensite, which is very hard and very brittle; tempering, reheating gently afterwards, trades some of that hardness back for toughness. Smiths were doing this reliably for two thousand years before anyone could say what was happening, and the metallurgical explanation arrived only in the late nineteenth and early twentieth centuries.

Cast iron, above about 2 per cent carbon, is brittle but pours well, and was made in China from around the fifth century BCE, more than a thousand years before it became practical in Europe. Wrought iron, nearly carbon-free, is tough and easily forged. The blast furnace, Henry Cort's puddling process of 1784, and above all Henry Bessemer's converter of 1856, which blew air through molten iron and burned out the carbon in minutes rather than days, took steel from a costly specialty to the cheapest structural material there is.

Adding chromium above about eleven per cent gives stainless steel, developed independently in several countries around 1912 and 1913, in which an invisible chromium oxide film reforms as fast as it is scratched away. Manganese, nickel, molybdenum, vanadium and tungsten each buy specific properties.

Iron rusts because its oxide flakes rather than sealing, unlike the oxides of aluminium or chromium. That single failure of chemistry costs the world economy a measurable share of output every year, and it is the reason for galvanising, for painting, and for much of the alloying above.